Fast recovery diode (FRD) is a kind of semiconductor diode with good switching characteristics and short reverse recovery time. It is mainly used as a high frequency rectifier diode, a freewheeling diode, or a damping diode in electronic circuits, such as a switching power supply, a PWM (pulse width modulator), or a frequency converter. The internal structure of the fast recovery diode is different from that of the ordinary PN junction diodes, the fast recovery diode belongs to a PIN junction diode. In other words, a base region I is added between P type silicon material and N type silicon material to form a PIN device. Because the base region I is very thin, the reverse recovery charge is very small, therefore, the reverse recovery time of the fast recovery diode is relatively short, the forward voltage drop of the fast recovery diode is relatively low, the reverse breakdown voltage (withstand voltage value) of the fast recovery diode is relatively high.
With the rapid development of the power devices such as IGBTs, the FRD device which is used with the IGBTs is becoming an indispensable device in power electronic systems. To the FRD power device that is used with high voltage IGBTs, its reverse breakdown voltage is as important as its reverse recovery characteristic and forward voltage drop, moreover, the breakdown voltage of the FRD device is lower than the breakdown voltage of the IGBTs would damage the IGBT devices in module.
FIG. 1 is a structural schematic diagram illustrating a fast recovery diode in the prior art, the device includes: a N− substrate 10, a P type doped field-limiting ring region 101, a P type doped anode region 102, a back implanted N+ doped region 103, a field oxide 40, and a first metal layer 50 and a second metal layers 60; wherein, the field-limiting ring region 101 and the anode region 102 are formed by doping on the N− substrate, the field oxide 40 grows on the surface of the substrate, the first metal layer 50 is deposited on the field oxide 40, and is finally formed as an anode electrode of the fast recovery diode, the N+ doped region 103 is formed on the backside of the substrate 10, the second metal layer 60 is deposited on the N+ doped region 103, and is formed as a cathode electrode.
The device structure adopts the field-limiting ring (FLR) and the field plate (FP) technology to increase the breakdown voltage of the FRD device. However in its fabrication process, a conventional planar process is adopted, by ion implantation and then annealing the field-limiting ring region 101 and the anode region 102 to form PN junction. The profile of the PN junction after the annealing process is shown in FIG. 1. As shown, curved parts 70 formed near the silicon surface, such curved parts 70 would make the curvature radius of the PN junction smaller, which would cause an accumulation of electric field at the curved parts, and a breakdown is likely to occur. This would result in a lower breakdown voltage and a poor reliability of the FRD device.
Therefore, the technical problem that the present invention needs to solve is to avoid the appearance of curved parts in the profile of the PN junction, so as to increase the breakdown voltage of the FRD device.